Method for applying a high-temperature bond coat on a metal substrate, and related compositions and articles

ABSTRACT

A method for applying a bond coat on a metal-based substrate is described. A slurry which contains braze material and a volatile component is deposited on the substrate. The slurry can also include bond coat material. Alternatively, the bond coat material can be applied afterward, in solid form or in the form of a second slurry. The slurry and bond coat are then dried and fused to the substrate. A repair technique using this slurry is also described, along with related compositions and articles.

[0001] This invention was made with government support under ContractNo. DEFC21-95-MC31176 awarded by the Department of Energy. Thegovernment may have certain rights to the invention.

BACKGROUND OF THE INVENTION

[0002] The invention disclosed herein generally relates to bond coatingsand thermal barrier coatings applied to metals. The metals arefrequently portions of components used in turbine engines. The inventionalso relates to processes for depositing such coatings.

[0003] Components formed of specialty materials like superalloys areused in various industrial applications, under a diverse set ofoperating conditions. In many cases, the components are provided withcoatings which impart several characteristics, such as corrosionresistance, heat resistance, oxidation resistance, and wear resistance.As an example, the various components of turbine engines, whichtypically can withstand in-service temperatures in the range of about1100° C.-1150° C., are often coated with thermal barrier coatings(TBC's), to effectively increase the temperature at which they canoperate.

[0004] Most TBC's are ceramic-based, e.g., based on a material likezirconia (zirconium oxide), which is usually chemically stabilized withanother material such as yttria. For a jet engine, the coatings areapplied to various superalloy surfaces, such as turbine blades andvanes, combustor liners, and combustor nozzles. Usually, the TBCceramics are applied to an intervening bond coating (sometimes referredto as a “bond layer” or “bond coat”) which has been applied directly tothe surface of the metal part. The bond coat is often critical forimproving the adhesion between the metal substrate and the TBC.

[0005] The effectiveness of a TBC is often measured by the number ofthermal cycles it can withstand before it delaminates from the substratewhich it is protecting. In general, coating effectiveness decreases asthe exposure temperature is increased. The failure of a TBC is oftenattributed to weaknesses or defects related in some way to the bondcoating, e.g., the microstructure of the bond coating. TBC failure canalso result from deficiencies at the bond coating-substrate interface orthe bond coating-TBC interface.

[0006] The microstructure of the bond coating is often determined by itsmethod of deposition. The deposition technique is in turn determined inpart by the requirements for the overlying protective coating. Forexample, many TBC's usually require a very rough bond coat surface(e.g., a root mean square roughness (R_(a)) value of greater than about200 micro-inches), for effective adhesion to the substrate. An airplasma spray (APS) technique is often used to provide such a surface.

[0007] There continues to be a need in the art for bond coatings whichprovide very good adhesion between the substrate and asubsequently-applied TBC, e.g., bond coatings with a relatively roughsurface. Furthermore, new processes for applying and curing suchcoatings in regions of a substrate which are somewhat inaccessible arealso of great interest. (Conventional thermal spray equipment issometimes too large and cumbersome for such regions). Moreover, theentire TBC system—bond coating with the TBC itself—should exhibit goodintegrity during exposure to high temperatures and frequent thermalcycles. Such a system should be effective in protecting components usedin high performance applications, e.g., superalloy parts exposed to hightemperatures and frequent thermal cycles.

SUMMARY OF THE INVENTION

[0008] One embodiment of the present invention is directed to a methodfor applying a bond coat on a metal-based substrate, comprising thefollowing steps:

[0009] a) applying a slurry which comprises braze material to thesubstrate, wherein the slurry also contains a volatile component;

[0010] b) applying bond coat material to the substrate;

[0011] c) drying the slurry and bond coat material under conditionssufficient to remove at least a portion of the volatile component; and

[0012] d) fusing the braze material and bond coat material to thesubstrate.

[0013] The braze material is usually based on nickel, cobalt, or iron.The bond coat material is often an “MCrAlX” material or a metal carbide,as discussed below.

[0014] There are a variety of methods for applying the bond coataccording to this invention. One method calls for combining the bondcoat material and the braze material with a solvent and one or moreadditives, as described below. The combined slurry mixture can then bedeposited on the substrate by various techniques, such as flow-coating,brushing, or spraying. As an alternative, the slurry applied in step (a)includes the braze material but not the bond coat material, and issubstantially dried to form a green layer. An adhesive can be applied tothe green layer, and the bond coat particles can then be applied to theadhesive, prior to the fusing step. As another alternative, two separateslurries can be employed—one containing the braze material, and theother containing the bond coat material. Each slurry can contain theadditives described below. In this embodiment, the braze slurry isusually applied first, followed by the application of the bond coatslurry. The slurries can then be dried and fused to the substrate. Anovercoat can optionally be applied over the bond coat. The overcoat isusually a conventional thermal barrier coating, e.g., one based onzirconium. Alternatively, the overcoat can be of another type, such as ametal carbide-based wear coating.

[0015] A method for replacing a bond coat applied over a metal-basedsubstrate is also described below. The following steps are usuallyincluded in this method:

[0016] (i) removing the existing bond coat from a selected area on thesubstrate;

[0017] (ii) applying a slurry which comprises braze material to theselected area, wherein the slurry also contains a volatile component;

[0018] (iii) applying additional bond coat material to the selectedarea; and

[0019] (iv) fusing the braze material and bond coat material to theselected area.

[0020] This technique can be part of the overall process for repairing aworn or damaged TBC system.

[0021] Another embodiment of this invention is directed to a uniqueslurry composition, containing a braze material and a bond coatmaterial, along with other conventional slurry ingredients, such as asolvent. As discussed elsewhere, the braze material is usually nickel,cobalt, iron, a precious metal, or some mixture containing one of thosecomponents. The bond coat material is usually of the MCrAlX-type(discussed below), or can be a metal carbide or other type of material.The slurry composition is very useful in the formation of a TBC system.

[0022] An article constitutes another embodiment of this invention. Itcomprises:

[0023] (a) a metal-based substrate, and

[0024] (b) a volatile-containing slurry on the substrate, comprisingbraze material and bond coat material (e.g. roughness-producing bondcoat particles).

[0025] The substrate is often a superalloy, and the braze material andbond coat materials are as described below. When the volatile componentin the slurry has been substantially removed, a green coating remains,which is fused to the substrate, e.g., by brazing. As fused, the brazematerial forms a continuous matrix phase in which the bond coatparticles are embedded.

[0026] Other features and advantages of the present invention will bemore apparent from the following detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a cross-sectional photomicrograph of a comparativecoating system which includes a bond coat and a TBC, both applied byconventional methods.

[0028]FIG. 2 is a cross-sectional photomicrograph of a bond coat/TBCcoating system, in which the bond coat was applied by a slurry techniqueaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0029] The braze material used in this invention can be formed of analloy composition which is known in the art and commercially available.Two classes of braze compositions are frequently used: standard liquidbrazes and activated diffusion brazes. Very often (but not always), thebraze alloy has a composition similar to that of the substrate. Forexample, if the substrate is a nickel-base superalloy, the braze alloyusually contains at least about 40% by weight nickel, along with variousother elements, like chromium, aluminum, and yttrium. (Nickel-containingbraze or cobalt-containing braze alloys are usually used withcobalt-base superalloys). The braze alloy composition also typicallycontains one or more components for lowering its melting point. Examplesof melting point suppressants for nickel-base and cobalt-base brazealloy compositions are silicon, boron, and phosphorous. Silicon orboron, or combinations thereof, are often preferred. The braze alloycomposition may also contain other additives known in the art, e.g.,fluxing agents. The average particle size of the braze alloy is usuallyin the range of about 20 microns to about 150 microns, and morepreferably, in the range of about 40 microns to about 80 microns.

[0030] Illustrative nickel-base and cobalt-base braze alloy compositionsare described in the commonly-assigned U.S. patent application Ser. No.09/444,737 (W. Hasz), filed on Nov. 23, 1999, and incorporated herein byreference. Some preferred nickel-base braze alloy compositions for thepresent invention comprise about 5 wt % to about 15 wt % silicon orboron; and about 15 wt % to about 25 wt % chromium, with the balancebeing nickel. Silicon is sometimes preferred over boron. Mixtures ofsilicon and boron are also possible.

[0031] Other types of braze alloys may be used, e.g., precious metalcompositions containing silver, gold, platinum, and/or palladium, incombination with other metals, such as copper, manganese, nickel,chromium, silicon, and boron. Mixtures which include at least one of thebraze alloy elements are also possible. Many of the metal brazecompositions are available from Praxair Surface Technologies, Inc.

[0032] As mentioned above, the braze material is utilized in the form ofa slurry. The slurry usually contains at least one binder and a solvent.Selection of the solvent depends on various factors, such as itscapacity for solubilizing the binder and dispersing the braze powder; aswell as the manner in which the slurry will be applied to the substrate.The braze material can usually be dispersed in either an aqueous ororganic solvent. Examples include water, ethanol or other alcohols;ketones, nitrile solvents (e.g., acetonitrile); ketone-type solventslike acetone; aromatic solvents like toluene, xylene, or xylenol; andcompatible mixtures thereof. Sometimes, a two-solvent system ispreferred, in which one solvent flash-evaporates, while the otherevaporates more slowly and provides leveling properties. (As used inthis disclosure, the term “volatile component” generally refers to thesolvent (or multiple solvents) used in the slurry. It should also beunderstood that the binders and other ingredients in the slurry willalso volatilize, or will decompose, as the temperature is raised, e.g.,as fusing temperatures are approached).

[0033] A variety of binder materials may be used in the slurry, e.g.,water-based organic materials such as polyethylene oxide and variousacrylics, or solvent-based binders. The slurry may also contain variousother additives, such as dispersants, wetting agents, deflocculants,stabilizers, anti-settling agents, thickening agents, plasticizers,emollients, lubricants, surfactants, anti-foam agents, and curingmodifiers. In general, the additives are each used at a level in therange of about 0.01% by weight to about 10% by weight, based on theweight of the entire slurry composition. Those skilled in the art candetermine the most effective level for any of the additives, withoutundue effort.

[0034] Conventional details related to the mixing of the slurry aredescribed in various references, such as U.S. Pat. 4,325,754, which isincorporated herein by reference. (Slurry compositions are alsocommercially available). A variety of techniques can be used to applythe slurry to the substrate. Examples include slip-casting, brushing,painting, dipping, flow-coating, roll-coating, spin coating, andspraying. Various texts are instructive in this regard, e.g., theKirk-Othmer Encyclopedia of Chemical Technology, 4th Edition, Vol. 5,pp. 606-619; as well as the Technology of Paints, Varnishes andLacquers, Edited by C. Martens, Reinhold Book Corporation, 1968. U.S.patent application Ser. No. 09/378,956 (D. Sangeeta et al, filed Aug.23, 1999 and commonly-assigned) also describes some aspects of slurrytechnology, and is incorporated herein by reference.

[0035] Various types of bond coat materials can be used in the presentinvention. Most are well-known in the art. “High-temperature” bond coatsare often (but not always) preferred. These are bond coats used inapplications where the substrate is exposed to service temperatures ofat least about 500° C., and more often, at least about 900° C. Veryoften, the bond coat material is of the MCrAlX type, where “M” can bevarious metals or combinations of metals, such as Fe, Ni, or Co; andwhere “X” is selected from the group consisting of Y, Ta, Si, Hf, Ti,Zr, B, C, and combinations thereof. (“X” is usually yttrium). Some ofthe preferred alloys of this type have a broad composition (in weightpercent) of about 17% to about 23% chromium; about 4% to about 13%aluminum; and about 0.1% to about 2% yttrium; with M constituting thebalance. In some embodiments, M is a mixture of nickel and cobalt,wherein the ratio of nickel to cobalt is in the range of about 10:90 toabout 90:10, by weight.

[0036] As alluded to earlier, other types of bond coat materials can beused. Non-limiting examples include aluminide, platinum-aluminide;nickel-aluminide; platinum-nickel-aluminide, and mixtures thereof.Moreover, mixtures of MCrAlX-type materials with metals such aszirconium or hafnium may also be used. Those skilled in the art will beable to select the most appropriate bond coat material, based on enduse, cost, processing method, and other considerations.

[0037] The size of the bond coat particles may vary somewhat, and isrelated in part to the desired roughness for the bond coat. Usually, thebond coat particles have an average particle size of at least about 45microns. In those instances in which a subsequent TBC is to be appliedby air plasma spray (which often requires a rough, underlying surface),the bond coat particles usually have an average particle size of atleast about 150 microns. In some preferred embodiments, the bond coatparticles have a size in the range of about 150 microns to about 300microns. Larger particle sizes may be used in some cases, e.g., whengreater roughness is desired. Sometimes, these particles are referred toherein as the “primary bond coat particles”, which provide theconventional type of roughness (R_(a)) discussed below.

[0038] In preferred embodiments, especially in situations where an airplasma-sprayed TBC is to be applied, the bond coat particles have aparticular shape. The shape is sufficient to produce micro-roughness inthe bond coat after the material has been fused to the substrate withthe braze. The micro-roughness is distinct from, and in addition to, theroughness (R_(a)) provided by the primary bond coat particles. (Theconventional roughness is usually measured by surface profilometry.)Micro-roughness is a fine-scale roughness and back-folding on theprimary particle. All embodiments of the present invention provide verygood adhesion for a subsequently-applied TBC. However, the presence ofthe micro-roughness greatly increases that adhesion in many instances,during the service life of the TBC.

[0039] There are several ways of obtaining micro-roughness, and theyusually involve the use of bond coat powder which is commerciallyavailable and known to provide such an effect. The micro-roughness maybe in the form of smaller spheres of the bond coat material (e.g.,having a size in the range of about 5% to about 50% of the primaryparticle diameter) which are bonded to the larger, primary particles.

[0040] Alternatively, the micro-roughness may be in the form of anirregular or rough surface on the primary particles. In this instance,the surface of the primary particles is convoluted and somewhat jagged,with undercut sections which appear to back-fold in some areas. Such aparticle surface has an appearance similar to that resulting when onewould tear an English muffin in half. Powder particles having thesecharacteristics (e.g., with an MCrAlX-type composition) are commerciallyavailable.

[0041] In one embodiment of this invention, the slurry also contains thebond coat material, so that the braze material and the bond coatmaterial are applied to the substrate simultaneously. Any convenienttechnique for combining the braze and bond coat materials in a singleslurry may be employed, e.g., mechanical mixers. In addition tofollowing general safety procedures, care should be taken to keep eachof the metal components well-dispersed in the slurry. At least oneaqueous or organic solvent is used for the slurry. Choice of aparticular solvent or solvent mixture for this embodiment will depend inpart on the solvent's compatibility with both the braze and bond coatmaterials, as well as with any melting point suppressant which may bepresent. The solvents should also be capable of maintaining the solidcomponents substantially dispersed. Moreover, the additives (mentionedabove) which are used in the slurry should be compatible with eachother, and with the other components in the slurry.

[0042] The slurry is usually deposited on the substrate as a singlelayer. However, in some instances, it may be desirable to deposit theslurry in the form of at least two “sub-layers”, i.e., in at least twoapplications. For example, each sub-layer may include the samecomposition, but the size of the bond layer particles may be varied.Smaller particles may be used in the sub-layer closest to the substrate,for increased coating density. Larger particles may be used in one ormore upper sub-layers, to provide a desired amount of roughness. (A heattreatment could be applied after the application of each sub-layer).

[0043] As another alternative, the composition of two or more sub-layerscan be varied, to provide different properties at different depths ofthe bond coat. As an example, a first sub-layer could contain a standardNiCrAlY-type bond coat material, along with the braze alloy. A secondsub-layer applied over the first sub-layer could contain braze alloy,along with a different bond coat material, e.g., an MCrAlX-type bondcoat material in which M is a mixture of nickel and cobalt. The secondsub-layer, which is closer to the atmosphere during service, shouldprovide greater corrosion resistance than the standard NiCrAlY materialin some environments.

[0044] In a similar fashion, the composition of two or more sub-layerscould be varied to adjust the degree to which oxidation occurs, e.g.,oxidation at the bond coat-substrate interface, as discussed in theexamples. Moreover, the composition of the bond coat could be graded orlayered, (e.g., by a metering system), so that the change in particularconstituents is gradually made as the slurry component is applied overthe substrate.

[0045] After the slurry mixture has been deposited, at least a portionof the volatile material contained therein is removed. This step issometimes referred to as an “evaporation step” or “evaporation stage”,and results in a substantially-devolatilized (solvent-free) coating,i.e., a “green” coating. Any convenient drying technique can be used toremove the volatile component. Drying may include air- or vacuum-dryingat room temperature. In some instances, it may be desirable to heat theslurry mixture to accelerate drying.

[0046] The green coating, which contains the braze material and the bondcoat material, is then fused to the substrate. The fusing step can becarried out by various techniques. Very often, it is a brazing step, andis similar to any conventional brazing operation. (As used herein,“brazing” is generally meant to include any method of joining metalsthat involves the use of a filler metal or alloy.) One exemplaryreference for details regarding brazing is the text Modem Metalworking,by J. R. Walker, The Goodheart-Willcox Co., Inc., 1965, pp. 29-1 to30-24. Those of ordinary skill in the art are familiar with otherdetails regarding brazing. Brazing temperatures depend in part on thetype of braze alloy used, and are typically in the range of about 525°C. to about 1650° C. In the case of nickel-based braze alloys, brazetemperatures are usually in the range of about 800° C. to about 1260° C.When possible, brazing is often carried out in a vacuum furnace. Theamount of vacuum will depend in part on the composition of the brazealloy. Usually, the vacuum will be in the range of about 10⁻¹ torr toabout

[0047] 10⁻⁸ torr. Furnace brazing also removes any volatile materials(e.g., the binder) remaining in the green coating. Volatile content canbe determined by a variety of techniques, such as differential thermalanalysis (DTA) and thermal gravimetric analysis (TGA).

[0048] In some cases, the slurry may have to be applied to an area whichdoes not lend itself to the use of a furnace. As an example, thecomponent itself may be too large for insertion into a furnace. In sucha case, alternatives are possible. For example, a torch or otherlocalized heating means can be used. These techniques are known in theart and briefly described in the commonly-assigned patent application ofW. Hasz, Ser. No. 09/444,737, mentioned above and incorporated herein byreference.

[0049] In an alternative embodiment, the slurry contains the brazematerial and any necessary additives, but does not contain the bond coatmaterial. In this instance, the slurry is substantially dried afterbeing applied, to form a green layer. Any convenient drying techniquecan be used, like those described above, e.g., air drying, prior to orfollowing an optional heat treatment to increase the evaporation of thevolatile component.

[0050] The bond coat material—usually in the form of dry powderparticles—is then applied over the green layer. Usually, an adhesive isapplied to a surface of the green layer, prior to the application of thebond coat powder. A variety of adhesives can be used, as long as theyare capable of completely volatilizing during the subsequent fusingstep. Some of the suitable adhesives are described, for example, in TheCondensed Chemical Dictionary, 10th Edition, B. Hawley, Van NostrandReinhold Company Inc., 1981, pp. 20-21, which is incorporated herein byreference. Illustrative examples of adhesives include polyethylene oxideand acrylic materials. Commercial examples of braze adhesives include“4B Braze Binder™”, available from Cotronics Corporation. The adhesivecan be applied by various techniques. For example, liquid-like adhesivescan be sprayed or coated onto the surface. A thin mat or film withdouble-sided adhesion could alternatively be used, e.g., 3M Company's467198 Adhesive Tape.

[0051] The bond coat powder can then be randomly applied over theadhesive by a variety of techniques, e.g., sprinkling, pouring, blowing,roll-depositing, and the like. After the deposition, the excess powderis removed from the substrate (e.g., by shaking or by being blown off),leaving substantially a single layer of bond coat particles. Asdescribed previously, the size of the particles depends in large part onthe degree of roughness required for the bond coat. The green coating(to which the braze material is attached) is then fused to thesubstrate, as described above. The resulting coating system issubstantially identical to that formed in the first embodiment.

[0052] In another alternative embodiment, the bond coat material can beused in the form of a second slurry, i.e., separate from the slurrywhich contains the braze material. The second slurry would be formedwith at least one solvent, i.e., a solvent compatible with theparticular bond coat composition. The slurry would also contain one ormore of the other additives described previously, e.g., binders,dispersants, and the like. Moreover, the slurry can be applied over thefirst slurry by any of the techniques described above, such as spraying.In preferred embodiments, some or all of the volatile component in thefirst slurry is removed before the application of the second slurry, toavoid bubbling. Removal of the volatiles is usually carried out byheating, as described above. Volatiles are then removed from the secondslurry in a same or similar manner, prior to the fusing step. Theresulting coating system is substantially identical to that formed inthe other embodiments.

[0053] As another alternative, the bond coat can be in the form of asecond slurry which is then pre-mixed with the first slurry. Theresulting pre-mixture can be applied to the substrate, prior to removalof the volatile component. Fusing is then carried out in the mannerdescribed previously.

[0054] In some embodiments of the present invention, an overcoat isapplied over the bond coat, after the bond coat material has been fusedto the substrate with the braze material. The overcoat is usually athermal barrier coating, but it could be any type of coating whichprovides environmental protection, i.e., protection of the substratefrom the adverse effects of oxidation, corrosion, or chemical attack.The overcoat could also be a wear-resistance coating. Moreover, theovercoat is usually ceramic, but could alternatively be metallic.

[0055] Ceramic thermal barrier coatings are often (but not always)zirconia-based. As used herein, “zirconia-based” embraces ceramicmaterials which contain at least about 50% zirconia, by weight. Zirconiais a well-known compound for barrier coatings. Its use is described, forexample, in Kirk-Othmer's Encyclopedia of Chemical Technology, 3rdEdition, V. 24, pp. 882-883 (1984). In preferred embodiments, thezirconia is chemically stabilized by being blended with a material suchas yttrium oxide, calcium oxide, magnesium oxide, cerium oxide, scandiumoxide, or mixtures of any of those materials. In one specific example,zirconia can be blended with about 1% by weight to about 20% by weightyttrium oxide (based on their combined weight), and preferably, fromabout 3%-10% yttrium oxide.

[0056] Various techniques can be used to apply the ceramic coating.Non-limiting examples include a thermal spray technique such as APS;physical vapor deposition (PVD); or electron beam physical vapordeposition (EB-PVD). Those of ordinary skill in the art are familiarwith the details regarding each of these deposition techniques. Relatedreferences include Kirk-Othmer's Encyclopedia of Chemical Technology,3rd Edition, Vol. 15, (1981) and Vol. 20 (1982); Ullmann's Encyclopediaof Industrial Chemistry, Fifth Edition; Volume A6, VCH Publisher (1986);Scientific American, H. Herman, September 1988; and U.S. Pat. 5,384,200.Ceramic slurry techniques or sol gel techniques can also be used toapply the ceramic coating.

[0057] Examples of other types of materials for the overcoat includewear-resistant coatings, e.g., carbide coatings such as chromium carbideand tungsten carbide, and those formed fromcobalt-molybdenum-chromium-silicon. Other types of material could beused as well, e.g., alumina, mullite, zircon, and glassy-type materialssuch as strontium-calcium-zirconate glass. Those of ordinary skill inthe art will be able to select the most appropriate material for a givenend use application. Methods for preparing and applying such materialsare those described above for the zirconia TBC's, or consist of othertechniques well-known in the art. Moreover, some of the overcoats can beprepared and applied in the form of a slurry over the bond coat, asmentioned above. Slurry-based overcoats are also described in thecommonly-assigned U.S. patent application of D. Sangeeta, Ser. No.09/557,393, filed on Apr. 24, 2000 and incorporated herein by reference.For example, the Summary of the Invention and other sections of thatpatent application are instructive.

[0058] Another embodiment of this invention is directed to a method forreplacing a bond coat previously applied to a metal-based substrate. Thereplacement of the bond coat is often a part of the overall process ofrepairing a worn or damaged TBC. Careful repair of a TBC “system” (bondcoat and TBC) is critical in preventing degradation of the substrate. Inthe case of a turbine engine component, for example, it may be necessaryto repair the coating while the turbine is in service, i.e., after itsdelivery from the manufacturing site. The process disclosed hereinprovides a means for rapidly repairing or replacing selected areas of anexisting TBC system, without having to completely remove the coatingsfrom the entire part. The process is especially useful for repairingcoatings which are situated in areas not easily accessible to otherrepair techniques.

[0059] The steps for replacing the bond coat usually comprise:

[0060] (i) removing the existing bond coat (and a worn or damagedovercoat, if present) from a selected area on the substrate;

[0061] (ii) applying a slurry which comprises braze material to theselected area, wherein the slurry also contains a volatile component;

[0062] (iii) applying additional bond coat material to the selectedarea; and

[0063] (iv) fusing the braze material and bond coat material to theselected area.

[0064] As described previously, a single slurry can be used, containingboth the braze material and the bond coat material. Alternatively, twoseparate slurries can be used. As another alternative, a braze slurrycan be applied and then dried, followed by the application of anadhesive layer. Bond coat material can then be applied to the adhesivelayer.

[0065] The slurry and bond coat material can be air-dried between steps(iii) and (iv). Heating means, such as an IR lamp, may be used toaccelerate removal of the volatile component. The fusing step is oftencarried out by using a torch or other portable heating apparatus.

[0066] In the case of a turbine engine component which includes thecoating being repaired, the heat evolved during engine operation may besufficient to remove the volatile component and carry out fusing step(iv). This means of heating and curing could in fact be postponed untila slurry-based overcoat has been applied, as described below.

[0067] An overcoat can then be applied over the bond coat, in thoseinstances in which an overcoat is being replaced. Usually, the overcoat(e.g., a TBC) will be applied by a thermal spray process in a repairsetting. Plasma spraying is one convenient technique. However, theovercoat can alternatively be applied over the bond coat in the form ofa slurry, as discussed above (Application Ser. No. 09/557,393 ofSangeeta). As mentioned earlier, turbine engine operation temperaturesmay be sufficient to remove all volatile material, fuse the brazematerial and bond coat to the substrate; and cure the overcoat, all in asingle step.

[0068] Another embodiment of this invention is a slurry compositionwhich comprises braze material and bond coat material. Such a slurry isuseful for applying a bond coat, as described above. Standard liquidbrazes or activated diffusion brazes can be used in the slurry. When theslurry is being applied to a nickel-base superalloy, the braze alloyusually contains at least about 40% by weight nickel, along with variousother elements, like chromium, aluminum, and yttrium. The averageparticle size of the braze alloy is usually in the range of about 20microns to about 150 microns, as mentioned above.

[0069] The bond coat material in the slurry is usually of the MCrAlXtype, as described previously. The size of the bond coat particles mayvary somewhat. They often have an average particle size of at leastabout 45 microns.

[0070] The choice of solvent for the slurry will depend in part on thesolid components contained therein, and on the manner in which it willbe applied to the substrate. Exemplary solvents are described above,along with binder materials and a variety of other additives, e.g.,dispersants, wetting agents, and stabilizers. The level of brazematerial and bond coat in the slurry will depend on various factors,such as the desired thickness of the bond coat; the solubility anddispersibility of the bond coat and braze materials in the solvent orsolvent mixture; and the manner in which the slurry will be applied.Usually, the slurry comprises about 20% by weight to about 50% by weightbraze material, and about 50% by weight to about 80% by weight bond coatmaterial, based on total slurry weight. The slurry typically containsabout 10% or less by weight solvent, and about 10% or less by weightbinder.

[0071] Still another embodiment of this invention is directed to anarticle, comprising:

[0072] (a) a metal-based substrate, e.g., one formed of a superalloy;and

[0073] (b) a volatile-containing slurry on the substrate, comprisingbraze material and bond coat material. The particular components whichmay be found in such a slurry have been described previously. When thevolatile component in the slurry has been substantially removed, a greenlayer remains. The green layer is then fused to the substrate, e.g., bya brazing technique. In preferred embodiments, the braze material formsa continuous matrix phase in which the bond coat particles are embedded.The size of the bond coat particles can be selected to cause them toprotrude beyond the matrix phase. In that instance, they serve as arelatively rough surface, e.g., one having an R_(a) of greater thanabout 200 micro-inches, and preferably, greater than about 300micro-inches. Such a surface provides excellent adhesion to asubsequently-applied ceramic layer. An article containing such a layer(e.g., a zirconia-based TBC) is also within the scope of this invention.

[0074] In order that those skilled in the art may better understand theinvention, the following examples are provided by way of illustration,and not by way of limitation.

EXAMPLE 1

[0075] Sample A was provided for the purpose of comparison, andrepresents a typical TBC system. The substrate was a coupon made from anickel-base superalloy. The coupon was grit-blasted and ultrasonicallycleaned. A NiCrAlY-type bond coat was then air plasma-sprayed (APS) ontothe substrate surface. The nominal bond coat composition was as follows:68 wt % Ni, 22 wt % Cr, 9 wt % Al, and 1 wt % Y. The thickness of thebond coat was in the range of about 5-8 mils (about 127-203 microns). Ithad an average roughness R_(a) of about 500 to about 900 micro-inches. ATBC (thermal barrier coating: yttria-stabilized zirconia, with 8 wt % byweight yttria) was then air plasma-sprayed over the bond coat. Thethickness of the TBC was in the range of about 10-12 mils (about 254-305microns).

[0076] Sample B represented an embodiment of the present invention. Aslurry was first prepared by adding the following components to acetone,under agitation:

[0077] (a) coarse NiCrAlY-type bond coat powder, having an approximatecomposition as follows: 68 wt % Ni, 22 wt % Cr, 9 wt % Al, and 1 wt % Y.The powder had an average particle size of −30 +100 mesh, i.e., 150-600microns.

[0078] (b) High-temperature braze powder, commercially available asAmdry® 100, having the following approximate composition: 10% by weightsilicon; 19% by weight chromium, base nickel. The powder had an averageparticle size of about-100 mesh, i.e., less than about 150 microns.

[0079] (c) Nicrobraz® 300 binder (ethyl methacrylate in trichloroethane;available from Wall Colmonoy, Inc., Madison Heights, Mich.)

[0080] The metal powders were dry-mixed (50 wt % of component (a) and 50wt % of component (b)). Components (c) and (d) were then added and mixed(10 wt % of each to the total slurry weight).

[0081] The slurry was applied (by brushing) to the same type ofsuperalloy coupon used for sample A. The wet thickness of the slurry wasabout 5 mils (127 microns). The slurry was then allowed to air-dry forabout 12 hours, during which at least about 15 wt % of the volatilematerial was removed. The resulting green coating was then heated in avacuum furnace at a brazing temperature of about 1093-1204° C.(2000-2200° F.), for about 1 hour. A dense, rough bond coat wasproduced, having an R_(a) of about 25 microns (about 984 micro-inches).The same type of thermal barrier coating (zirconia-based) used in sampleA was then air plasma-sprayed over the bond coat.

[0082]FIG. 1 is a cross-sectional photomicrograph of the coating systemfor sample A. Region I is the substrate. Region II is an oxide regionwhich has begun to form between the substrate and the bond coat, as aresult of thermal testing. Region III is the bond coat itself,exhibiting the typical overlapping of bond coat material “splats” whichresult from APS deposition. Region IV is the TBC.

[0083] The overall coating system of sample A exhibits good integrityfor some end uses, and for a projected service life. However, Region IIresults from accelerated oxidation at the bond coat-substrate interface,toward the end of a simulated service life. The oxidation willeventually result in coating failure, by causing most or all of the TBCand bond coat to become detached from the substrate.

[0084]FIG. 2 is a cross-sectional photomicrograph of the coating systemfor sample B, prepared according to an embodiment of the presentinvention. The coating system was subjected to the same amount ofthermal testing as sample A. Region V is the substrate. Region VI is theslurry layer applied and brazed over the substrate. Region VII is theTBC. The absence of the oxidation region seen in FIG. 1 indicates thataccelerated oxidation at the bond coat-substrate interface has not takenplace.

[0085] The type of thermal testing carried out for each sample wasFurnace Cycle Testing (FCT). One cycle represented 45 minutes at 2000°F. (1093° C.). The process continued for 300 cycles for each sample. Theresults demonstrated that the present invention (sample B) had a furnacecycle life which was about 3 times greater than that of the comparativebase line sample (sample A).

EXAMPLE 2

[0086] In this example, the same type of substrate was used, to preparesample C. A slurry was prepared, containing 80 wt % of the Amdry® 100high-temperature braze powder, along with 10 wt % water and 10 wt %polyethylene oxide binder. The slurry was applied to the substrate, to awet thickness of about 5 mils (127 microns). The slurry was then allowedto dry for about 14-16 hours. About 15 wt % of the volatile material wasremoved in this drying step, leaving a green layer. A layer of brazecontact adhesive (Nicrobraz™ 300) was then applied over the green layer.

[0087] The coarse NiCrAlY-type bond coat powder used in Example 1(sample B) was then sprinkled onto the contact adhesive, to produce amono-layer of coarse powder. The excess bond coat powder was blown off.The coupon was then heated in a vacuum furnace at a brazing temperatureof about 1093-1204° C. (2000-2200° F.), for about 0.25-2 hours, toproduce a dense, rough bond coat (R_(a) of about 25 microns/984micro-inches). The same type of thermal barrier coating (zirconia-based)used in sample A was then air plasma-sprayed over the bond coat.

[0088] FCT tests were carried out, as in Example 1. Sample C displayedapproximately the same properties (resistance-to-cracking anddelamination) as those of sample B. Moreover, sample C showed noindication of the accelerated oxidation at the bond coat-substrateinterface, i.e., as evidenced for sample A.

[0089] Preferred embodiments have been set forth for the purpose ofillustration. However, the foregoing description should not be deemed tobe a limitation on the boundaries of the invention. Accordingly, variousmodifications, adaptations, and alternatives may occur to one skilled inthe art without departing from the spirit and scope of the presentinvention.

[0090] All of the patents, articles, and texts mentioned above areincorporated herein by reference.

What is claimed:
 1. A method for applying a bond coat on a metal-basedsubstrate, comprising the following steps: a) applying a slurry whichcomprises braze material to the substrate, wherein the slurry alsocontains a volatile component; b) applying bond coat material to thesubstrate; c) drying the slurry and bond coat material to remove atleast a portion of the volatile component; and d) fusing the brazematerial and bond coat material to the substrate.
 2. The method of claim1, wherein the braze material comprises at least one metal selected fromthe group consisting of nickel, cobalt, iron, a precious metal, and amixture which includes at least one of the foregoing.
 3. The method ofclaim 2, wherein the braze material comprises at least about 40% byweight nickel.
 4. The method of claim 2, wherein the braze materialfurther comprises a constituent which lowers the melting point of thebraze alloy.
 5. The method of claim 4, wherein the constituent issilicon, boron, or mixtures thereof.
 6. The method of claim 1, whereinthe braze material has an average particle size in the range of about 20microns to about 150 microns.
 7. The method of claim 1, wherein theslurry further comprises at least one additive selected from the groupconsisting of binders, stabilizers, thickening agents, dispersants,deflocculants, anti-settling agents, plasticizers, emollients,lubricants, surfactants, anti-foam agents, and curing modifiers.
 8. Themethod of claim 1, wherein the slurry is applied to the substrate by atechnique selected from the group consisting of slip-casting, brushing,painting, dipping, flow-coating, roll-coating, spin coating, andspraying.
 9. The method of claim 1, wherein the bond coat materialcomprises an alloy of the formula MCrAlX, where M is selected from thegroup consisting of Fe, Ni, Co, and mixtures of any of the foregoing;and where X is selected from the group consisting of Y, Ta, Si, Hf, Ti,Zr, B, C, and combinations thereof.
 10. The method of claim 1, whereinthe bond coat comprises a material selected from the group consisting ofaluminide, platinum-aluminide; nickel-aluminide;platinum-nickel-aluminide; and mixtures thereof.
 11. The method of claim1, wherein the bond coat material has an average particle size of atleast about 45 microns.
 12. The method of claim 11, wherein the bondcoat material has an average particle size in the range of about 150microns to about 300 microns.
 13. The method of claim 1, wherein thevolatile component comprises at least one aqueous solvent or at leastone organic solvent, or mixtures thereof.
 14. The method of claim 1,wherein step (c) is carried out by air-drying.
 15. The method of claim1, wherein step (d) is carried out at a temperature in the range ofabout 525° C. to about 1650° C.
 16. The method of claim 1, wherein themetal-based substrate is a superalloy.
 17. The method of claim 16,wherein the superalloy is a nickel-base or cobalt-base material.
 18. Themethod of claim 1, wherein the bond coat has a root mean squareroughness (R_(a)) value of greater than about 200 micro-inches, afterstep (d).
 19. The method of claim 1, wherein the slurry also containsthe bond coat material, so that the braze material and the bond coatmaterial are applied to the substrate simultaneously.
 20. The method ofclaim 19, wherein the slurry is prepared by combining the bond coatmaterial and the braze material with a solvent and at least one additiveselected from the group consisting of binders, stabilizers, thickeningagents, dispersants, deflocculants, anti-settling agents, plasticizers,emollients, lubricants, surfactants, anti-foam agents, and curingmodifiers.
 21. The method of claim 19, wherein the braze material has anaverage particle size in the range of about 20 microns to about 150microns, and the bond coat material has an average particle size of atleast about 45 microns.
 22. The method of claim 1, wherein the slurry ofstep (a) is substantially dried after being applied, thereby forming agreen layer, prior to the application of the bond coat material.
 23. Themethod of claim 22, wherein an adhesive material is applied to the greenlayer, prior to the application of the bond coat material.
 24. Themethod of claim 23, wherein the bond coat material is in the form of dryparticles, and is applied to the adhesive material.
 25. The method ofclaim 24, wherein the bond coat material is poured or sprinkled onto theadhesive.
 26. The method of claim 1, wherein the bond coat material isin the form of a second slurry, applied in step (b) on top of the slurryapplied in step (a).
 27. The method of claim 26, wherein each slurrycomprises at least one additive selected from the group consisting ofbinders, stabilizers, thickening agents, dispersants, deflocculants,anti-settling agents, plasticizers, emollients, lubricants, surfactants,anti-foam agents, and curing modifiers.
 28. The method of claim 26,wherein each slurry is applied to the substrate by a technique selectedfrom the group consisting of slip-casting, brushing, painting, dipping,flow-coating, roll-coating, spin coating, and spraying.
 29. The methodof claim 1, wherein the bond coat is in the form of a second slurry, andthe second slurry is pre-mixed with the first slurry to form apre-mixture, said pre-mixture being applied to the substrate prior tostep (c).
 30. The method of claim 29, wherein the pre-mixture is appliedto the substrate by a technique selected from the group consisting ofslip-casting, brushing, painting, dipping, flow-coating, roll-coating,spin coating, and spraying.
 31. The method of claim 1, wherein anovercoat is applied over the bond coat after step (d).
 32. The method ofclaim 31, wherein the overcoat is a thermal barrier coating.
 33. Themethod of claim 32, wherein the thermal barrier coating iszirconia-based.
 34. The method of claim 32, wherein the thermal barriercoating is applied by a thermal spray technique.
 35. The method of claim34, wherein the thermal spray technique is a plasma spray process. 36.The method of claim 31, wherein the overcoat is a wear-resistantcoating.
 37. The method of claim 31, wherein the overcoat comprises amaterial selected from the group consisting of metal carbides; alumina,mullite, zircon, cobalt-molybdenum-chromium-silicon;strontium-calcium-zirconate glass; and mixtures thereof.
 38. A methodfor applying a metal aluminide- or MCrAlX-based bond coat on asuperalloy substrate, where M is nickel, cobalt, or a mixture thereof,comprising the following steps: (I) applying a slurry which comprises avolatile component and a mixture of braze material and bond coatmaterial to the substrate, wherein the braze material contains at leastabout 40% by weight nickel; (II) drying the slurry under conditionssufficient to remove at least a portion of the volatile component,forming a green coating; and (III) brazing the green coating to thesubstrate.
 39. The method of claim 38, wherein the braze material has anaverage particle size in the range of about 40 microns to about 80microns, and the bond coat material has an average particle size in therange of about 150 microns to about 300 microns.
 40. The method of claim38, wherein a zirconia-based thermal barrier coating is applied over thebond coat.
 41. A method for replacing a bond coat applied over ametal-based substrate, comprising the following steps: (i) removing theexisting bond coat from a selected area on the substrate; (ii) applyinga slurry which comprises braze material to the selected area, whereinthe slurry also contains a volatile component; (iii) applying additionalbond coat material to the selected area; and (iv) fusing the brazematerial and bond coat material to the selected area.
 42. The method ofclaim 41, wherein the slurry also contains the bond coat material, sothat the braze material and the bond coat material are applied to thesubstrate simultaneously.
 43. The method of claim 41, wherein the bondcoat material comprises an alloy of the formula MCrAlX, where M isselected from the group consisting of Fe, Ni, Co, and mixtures of any ofthe foregoing; and where X is selected from the group consisting of Y,Ta, Si, Hf, Ti, Zr, B, C, and combinations thereof.
 44. The method ofclaim 41, wherein the metal-based substrate is a portion of a turbineengine.
 45. A slurry composition, comprising a braze material and a bondcoat material.
 46. The composition of claim 45, wherein the brazematerial comprises at least one metal selected from the group consistingof nickel, cobalt, iron, a precious metal, and a mixture which includesat least one of the foregoing.
 47. The composition of claim 46, whereinthe braze material comprises at least about 40% by weight nickel. 48.The composition of claim 46, wherein the bond coat material comprises analloy of the formula MCrAlX, where M is selected from the groupconsisting of Fe, Ni, Co, and mixtures of any of the foregoing; andwhere X is selected from the group consisting of Y, Ta, Si, Hf, Ti, Zr,B, C, and combinations thereof.
 49. The composition of claim 46, whereinthe bond coat comprises a material selected from the group consisting ofaluminide, platinum-aluminide, nickel-aluminide;platinum-nickel-aluminide; and mixtures thereof.
 50. The composition ofclaim 46, wherein the bond coat material is in the form of particles,and the particles have an average size of at least about 45 microns. 51.An article, comprising: (a) a metal-based substrate; and (b) avolatile-containing slurry on the substrate, comprising braze materialand bond coat material.
 52. The article of claim 51, wherein thesubstrate is a superalloy.
 53. The article of claim 51, wherein thesubstrate is a component of a turbine engine.
 54. The article of claim51, wherein the braze material comprises at least about 40% by weightnickel, and the bond coat material comprises an alloy of the formulaMCrAlX, where M is selected from the group consisting of Fe, Ni, Co, andmixtures of any of the foregoing; and where “X” is selected from thegroup consisting of Y, Ta, Si, Hf, Ti, Zr, B, C, and combinationsthereof.
 55. The article of claim 51, wherein the volatile component inthe slurry has been substantially removed, so as to form a greencoating.
 56. The article of claim 55, wherein the green coating is fusedto the substrate, forming a bond coat.
 57. The article of claim 56,wherein the braze material forms a continuous matrix phase in which thebond coat particles are embedded.
 58. The article of claim 57, whereinan overcoat is disposed over the bond coat.
 59. The article of claim 58,wherein the overcoat is a zirconia-based TBC.
 60. The article of 58,wherein the overcoat is a wear-resistant coating.
 61. An article,comprising: (i) a metal-based substrate, and (ii) a brazed bond coatcomprising bond coat particles, disposed over the substrate, wherein thebraze material forms a continuous matrix phase in which the bond coatparticles are embedded.
 62. The article of claim 61, wherein the brazedbond coat has a root mean square roughness (R_(a)) value of greater thanabout 200 micro-inches.
 63. The article of claim 61, wherein the brazedbond coat is characterized by micro-roughness.
 64. The article of claim61, wherein the brazed material is nickel-based or cobalt-based.
 65. Thearticle of claim 61, wherein the bond coat is a higher-temperaturematerial.
 66. The article of claim 61, wherein the bond coat particlesare roughness-producing.